1. (Field of the Invention)
The present invention relates to a diagnostic ultrasound apparatus, and in particular, a diagnostic ultrasound apparatus capable of performing imaging known as flash echo imaging (FEI) involving the injection of an ultrasound contrast agent into an object.
2. (Description of Prior Art)
An ultrasound diagnostic apparatus has now become an indispensable modality in clinical sites, because, in addition to display images in real time, they have advantages of relatively low in cost, no exposure of X-rays, and allowing blood flow imaging based on an ultrasound Doppler technique.
Particularly, this blood flow imaging, which is a function that is effective in finding lesions in the cardiac system or others, is known as a technique of color flow mapping CFM or color Doppler tomography and is provided as a standard option in most apparatuses.
As widely known, this display requires that the same raster location (direction) of an object be ultrasound-scanned a plurality of N times to acquire time-sequential echo signals and those echo signals undergo the detection of velocities of blood cells at a desired depth position based on the Doppler technique. That is, obtaining a Doppler signal requires the same raster location to be scanned repeatedly at certain intervals. Based on phase shift amounts (Doppler shift amounts) of per unit time obtained from a reflected signal from the blood cells, blood flow velocities can be obtained.
An echo signal resulting from each time of ultrasound scanning contains a reflection wave from objects in motion such as blood cells and a reflection wave from fixed objects almost stationary, such as a blood vessel wall or organic parenchyma. Additionally it is characteristic that the latter is a dominant in terms of reflection intensity, and further, Doppler shifts are caused in the former but not almost caused in the latter (clutter signal). Thus, a Doppler signal is extracted from the echo signal by a quadrature phase detector, and a clutter component is eliminated by an MTI filter from the Doppler signal on the basis of differences in the Doppler shifts, thereby a blood flow Doppler signal being detected efficiently. This blood flow Doppler signal is then subjected to frequency analysis with its N-piece Doppler data at each depth position, thereby a mean of its spectrum (Doppler frequency), dispersion amount, and/or intensity (power) reflected from blood cells being calculated. These pieces of blood flow information are two-dimensionally displayed on a monitor, normally, with a B-mode image placed as a background.
Recently, a technique has eagerly been tried that an ultrasound contrast agent is injected into an object""s blood vessel to enhance scattering intensity of ultrasound for improving a diagnostic performance. Particularly, the performances of the contrast agent have noticeably been improved for the fast few years, such that contrast effects have been improved and the agent has been allowed to be injected from the vein, resulting in reduced invasiveness. It is expected that this kind of contrast agent become popular more. In association with this, there has been a need that a diagnostic ultrasound apparatus should have capability of performing diagnosis with making use of all the characters of the contrast agent that has been improving year by year. The background of this need will now be detailed.
An ultrasound contrast agent now under development consists mainly of a few microns of microbubbles. It has been known that these microbubbles easily collapse at almost as similar sound pressures as used for ordinary ultrasound diagnosis, and generates a higher-intensity harmonic corresponding to a harmonic of an ultrasound pulse when the collapse occurs, so that showing a higher contrast effect. An imaging technique called flash echo imaging (FEI) obtaining B-mode images utilizing this character is reported by, for example, a paper xe2x80x9cJapanese Journal of Medical Ultrasonics, Vol.23, Supplement 1; June, 1996; 67-195,67-196xe2x80x9d (the first report). This paper reports that, after a full charge of microbubbles by temporarily stopping ultrasound radiation, re-radiating an ultrasound pulse will cause large amounts of luminance enhancement in a tomographic image (tomographic image based on a harmonic component) simultaneously with the radiation, then the echo diminish immediately after that enhancement. This phenomenon is referred to as a flash echo phenomenon.
As to a prior art reference between color flow mapping and an ultrasound contrast agent, a paper xe2x80x9cJapanese Journal of Medical Ultrasonics, Vol.22, Supplement 2; November, 1995; 66-33xe2x80x9d is reported (the second report). This paper reports that, when an operator operates a freezing button to temporarily stop to transmit an ultrasound wave into the lever of a rabbit into which an ultrasound contrast agent was injected, then starts again the scanning with operating the freezing button, which provides the on/off states of transmission, a mosaic color image can be obtained in a velocity mode of color flow mapping. In this report, an image on this phenomenon is called cavitation image. Also reported is that this color image cannot be obtained at sound pressures less than a certain amount.
It is considered that this cavitation image can be obtained due to the same phenomenon as the flash echo image. Namely, considered is that this paper shows the fact that the flash echo phenomenon occurs with the fundamental wave of an ultrasound pulse and be observable using the color flow mapping. This also suggests that tissue blood (perfusion), which could not be imaged by the conventional color flow mapping, can be imaged.
Furthermore, the fact that the flash echo phenomenon is observable with even harmonic CFM images is reported by a paper xe2x80x9cJapanese Journal of Medical Ultrasonics, Vol.23, Supplement 2; November, 1996; 68-156xe2x80x9d (the third report). Described in this paper is that, when the flash echo phenomenon occurs, a mosaic color image is obtained in the velocity mode of color flow mapping with a harmonic. Also reported by this paper is that a Doppler spectrum of a harmonic on the flash echo phenomenon is broad as shown in FIG. 21.
This means that the perfusion can be imaged as well and its color image is shown in a mosaic. In other words, since Doppler shifts seldom occur in a clutter component that is an echo reflected by organic parenchyma, this component can be eliminated by an MTI filter, but a harmonic on the flash echo phenomenon passes the MTI filter and is imaged in the end, even if the contrast agent is in motion (for example, in a blood vessel) or at rest (for example, perfusion), because of a broad band of its Doppler spectrum. Additionally, an average of a Doppler spectrum at each of spatial points differ from each other at random, resulting in a mosaic image.
By the way, taking the similarity between the above second and third reports into account, it is assumed that the Doppler spectrum of a fundamental wave on the flash echo phenomenon is also broad in band.
Further, another report on the contrast agent is made by papers xe2x80x9cJapanese Journal of Medical Ultrasonics, Vol.23, Supplement 2; November, 1996; 68-157xe2x80x9d and xe2x80x9cJournal of Medical Ultrasonics, Vol.24, No.3; March, 1997; 69P3-5xe2x80x9d (the fourth report). These reports explain spectrums on the flash echo phenomenon occurring in three types of contrast agent. From these spectrums reported, it has been found that the fundamental wave is higher than the harmonics in sensitivity given when the flash echo phenomenon occurs.
Further, there has been known that the intensity of an echo signal on the flash echo phenomenon reduces gradually with time until the microbubbles collapse entirely.
A summary from a clinical point of view can be given as follow: (1) Using the flash echo phenomena enables an ultrasound contrast agent to be detected highly sensitively, (2) the CFM method is usable for observing perfusion, and (3) the CFM method is visualized in color so as to be easy to understand, while the velocity mode is visualized as a mosaic color image so as to be easy for discrimination. From a technical point of view, (4) causing the flash echo phenomenon requires that the transmission of an ultrasound wave be halted temporarily, (5) the flash echo phenomenon will not basically occur at sound pressures smaller than a certain amount, (6) a signal on the flash echo phenomenon will diminish gradually in intensity until the microbubbles collapse entirely, (7) using the CFM method, the flash echo phenomenon is observable with either a fundamental wave or a harmonic, and (8) a Doppler spectrum shown when the flash echo phenomenon is caused is broad in band.
Thus, it is supposed that causing the flash echo phenomenon from a contrast agent injected into an object before producing the enhanced signals into a color-mapping image leads to supply of clinically effective images.
However, in the conventional diagnostic ultrasound apparatus making use of the flash echo phenomenon, an ultrasound imaging technique that maximally shows the priorities of this phenomenon has not been established yet. Particularly, as to not only setting of an imaging interval during which the flash echo phenomenon is caused and a non-imaging interval but also image display techniques during those intervals, any effective measure has not been proposed yet.
Further, in the conventional diagnostic ultrasound apparatus having a function of color flow mapping, to cause the flash echo phenomenon requires that a freezing button be operated by hand to temporarily halt the transmission, then to release this frozen state after a certain time. This manual operation becomes complicated and it is difficult to establish accurate on/off intervals, thus being lowered in practicality. As a result, this manual setting is not adopted into actual fields of clinics at all.
On one hand, there is not always guarantee that various types of signal processing which have already been performed with the conventional color flow mapping are matched to the detection and processing of an enhanced signal due to the flash echo phenomenon. There is even a possibility that the detection has not been carried out so sensitively.
Namely, although it has been thought that color flow mapping imaging of an enhanced signal on the flash echo phenomenon might be effective, there has not been provided an apparatus that has a capability to display images, maneuverability, and detection sensitivity raised to the extent necessary to make use of the practicality of this imaging in actual clinical fields.
The present invention has been made in consideration of the drawbacks of the foregoing prior art techniques. A first object of the present invention is to, for imaging in color flow mapping a reflection signal associated with the flash echo phenomenon caused by a contrast agent injected into an object, be able to suitably determine one interval during which the flash echo phenomenon is caused and the other interval during which it is not caused, and be able to obtain clinically effective images through those intervals.
A second object of the present invention is, in addition to the above first object, to perform processing steadily appropriate for a reflected signal owing to the flash echo phenomenon, so that the signal can be detected highly sensitively.
In order to accomplish the above objects, a diagnostic ultrasound apparatus of the present invention, as one mode, is provided for obtaining an image of an object into which an ultrasound contrast agent is injected, and the apparatus features that it comprises: an ultrasound transducer for transmitting and receiving the ultrasound wave to and from the object; transmitting means (or a unit) for transmitting an ultrasound pulse into the object by driving the ultrasound transducer under a first transmitting condition exerting a given destruction capability on the ultrasound contrast agent and a second transmitting condition having a destruction capability lower than that of the first condition; first image producing means (or a unit) for receiving an ultrasound echo of the ultrasound pulse transmitted under the first transmitting condition and producing a first color-displayed image based on phase displacement information about the ultrasound echo; second image producing means (or a unit) for receiving an ultrasound echo of the transmitted ultrasound pulse and producing a second image based on amplitude information about the ultrasound echo; and means (or a unit) for displaying both the first and second images.
As another mode, it is featured that a diagnostic ultrasound apparatus for obtaining an image of an object into which an ultrasound contrast agent is injected, the apparatus comprises: an ultrasound transducer for transmitting and receiving the ultrasound wave to and from the object; transmitting means (or a unit) for transmitting an ultrasound pulse into the object by driving the ultrasound transducer under a first transmitting condition exerting a given destruction capability on the ultrasound contrast agent and a second transmitting condition having a destruction capability lower than that of the first condition; first image producing means (or a unit) for receiving an ultrasound echo of the ultrasound pulse transmitted under the first and second transmitting conditions and producing a first color-displayed image based on phase displacement information about the ultrasound echo; second image producing means (or a unit) for receiving the ultrasound echo and producing a second image based on amplitude information about the ultrasound echo; and means (or a unit) for displaying both the first and second images.
Still as another mode, it is featured that a diagnostic ultrasound apparatus for obtaining an image of an object into which an ultrasound contrast agent is injected, the apparatus comprises: an ultrasound transducer for transmitting and receiving the ultrasound wave to and from the object; transmitting means (or a unit) for transmitting an ultrasound pulse into the object by driving the ultrasound transducer under a first transmitting condition exerting a given destruction capability on the ultrasound contrast agent and a second transmitting condition having a destruction capability lower than that of the first condition; first image producing means (or a unit) for receiving an ultrasound echo of the transmitted ultrasound pulse and producing a first color-displayed image based on phase displacement information about the ultrasound echo; second image producing means (or a unit) for receiving an ultrasound echo of the ultrasound pulse transmitted under the first transmitting condition and producing a second image based on amplitude information about the ultrasound echo; and means (or a unit) for displaying both the first and second images.
Still as another mode, it is featured that diagnostic ultrasound apparatus for obtaining an image of an object into which an ultrasound contrast agent is injected, the apparatus comprises: an ultrasound transducer for transmitting and receiving the ultrasound wave to and. from the object; transmitting means (or a unit) for transmitting an ultrasound pulse into the object by driving the ultrasound transducer at changeable intervals under a transmitting condition exerting a given destruction capability on the ultrasound contrast agent; image producing means (or a unit) for receiving an ultrasound echo of the ultrasound pulse transmitted under the transmitting condition and producing the image based on both phase displacement information and amplitude information about the ultrasound echo; and display means (or a unit) for displaying the image.
According to the above configurations, a contrast agent is injected and an ultrasound pulse is transmitted via the probe under the first and second transmitting conditions. Under the first transmitting condition, microbubbles of the contrast agent, which are essential constituents, are collapsed to cause a flash echo phenomenon, thus providing an echo signal in which the phenomenon is strongly reflected. In contrast, under the second transmitting condition, the microbubbles are basically collapsed, thus providing an echo signal in which only the tissue and/or blood flows are influenced. Thus appropriate changeover and setting of those transmitting conditions allow the on- and off-states of the flash echo phenomenon to be controlled.
In producing images under the first transmitting condition, because an echo signal contains a high-intensity reflection signal, called a clutter, originated from organic parenchyma, the clutter is removed from the echo signal using means for discriminating a signal from the contrast agent, providing only a flash echo signal caused by the contrast agent. The signal from the contrast agent may be a signal containing a fundamental signal from the contrast agent or a harmonic signal from the contrast agent. The flash echo signal emanated from the contrast agent, which is obtained by the discrimination, is directly displayed or displayed as information obtained by calculating Doppler frequency (velocity), dispersion, power and/or others from the flash echo signal of the contrast agent. The transmission and reception is automatically controlled, resulting in that CFM images of the flash echo signal can be obtained readily to provide a useful diagnostic ultrasound apparatus.
As another aspect, under the second transmitting condition, monitoring transmission and reception are performed with a lower sound pressure determined not to cause a flash echo phenomenon, and these scanning results are displayed in a wide variety of modes. For instance, B-mode tomographic images and/or CFM images can be displayed. Alternatively, the transmission and reception can be stopped under the second transmitting condition.
Further, through an interval during which the flash transmission is an on-state under the first transmitting condition and a monitor interval during which the flash transmission is an off-state under the second transmitting condition, B-mode tomographic images and/or CFM images can be displayed. This permits CFM images of a flash echo signal to be obtained while monitoring an object in real time. Accordingly, image information about a region to be diagnosed can be obtained during the off-interval of the flash transmission. In addition, a position of a region to be examined is easily decided and positions are resistant to being shifted even when waiting is done without performing flash echo imaging. In this way, a flash echo contrast agent examination can be performed while observing a region to be diagnosed in real time, and it is possible to provide a diagnostic ultrasound apparatus which can be used in a simple manner, with an excellent maneuverability, and with an improved diagnostic performance.
When a flash echo phenomenon occurs, the intensity of a flash echo signal begins to weaken little by little immediately after the occurrence. Thus, where the transmission and reception are performed a plurality of times along the same direction, the use of data obtained in an earlier stage of the data acquisition, which are higher in the intensity of a flash echo signal, leads to a higher sensitivity. In the case that means for discriminating the signal from the contrast agent use, for example, an MTI filter (clutter elimination means), Doppler signals acquired by a plurality of times of the transmission and reception are inputted into the MTI filter in the opposite order to that in the reception. This allows the data obtained in the earlier stage to be used, thereby improving the sensitivity.
Alternatively, means for discriminating the signal from the contrast agent are able to use means for calculating differences of signals adjoining to each other among a plurality of signals received through a plurality of times of the transmission and reception, a clutter component can be removed with signals obtained by at least two times of transmission and reception. This results in that data obtained in an earlier stage of the data acquisition can be used, thus increasing sensitivity.
Additionally, the transmission and reception for tomographic images can be conducted together with the transmission and reception for CFM images, so a CFM image of flash echo signals is superposedly represented on a tomographic image thus obtained. In this case, the transmission and reception of an ultrasound pulse for a signal from a contrast agent always precedes that for tomographic images. Thus it is avoided that higher-sensitivity data are consumed by only the transmission and reception of an ultrasound pulse for tomographic images. CFM images (blood flow color images) are also formed by using earlier-stage data, thus increasing sensitivity.
As described above, the present invention adopts a way of adapting various types of processing performed in the CFM technique to the flash echo imaging, so that a higher-sensitivity diagnostic ultrasound apparatus is provided in the examination with contrast agents.